Glass sheets and other substrates can be coated with a stack of transparent, metal-containing films to vary the properties of the coated substrates. Particularly desirable are coatings characterized by their ability to transmit visible light while minimizing the transmittance of other wavelengths of radiation, especially radiation in the infrared spectrum. These characteristics are useful for minimizing radiative heat transfer while controlling visible transmission. Coated glass of this nature is useful as architectural glass and as automotive glass.
Coatings having the characteristics of high visible transmittance and low emissivity typically include one or more infrared-reflective films and two or more antireflective transparent dielectric films. The infrared-reflective films reduce the transmission of radiant heat through the coating. The infrared-reflective films commonly are conductive metals (e.g., silver, gold, or copper), although transparent conductive oxides (e.g., ITO) or conductive nitrides (e.g., TiN) may also be used. The transparent dielectric films are used primarily to reduce visible reflection, to provide mechanical and chemical protection for the sensitive infrared-reflective films, and to control other optical coating properties, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium, as well as nitrides and oxynitrides of silicon. Low-emissivity coatings can be deposited on glass sheets through the use of well-known magnetron sputtering techniques.
It is known to deposit a thin metallic layer directly over an infrared-reflective silver film to protect the silver film during deposition of a subsequent dielectric layer and/or during tempering or any other heat treatment. These protective layers (sometimes called “sacrificial layers” or “blocker layers”) have been formed of various materials, such as titanium, niobium, niobium-titanium, or NiCr.
The particular material from which the blocker layer(s) is formed impacts various properties and characteristics of the coating. Titanium blocker layers, for example, have been found to impart excellent scratch resistance in low-emissivity coatings. They also adhere well to both an underlying silver film and an overlying oxide film. Niobium has been found to be an advantageous blocker layer material as well. In addition, niobium-titanium has been found to be particularly beneficial in certain respects.
It is sometimes necessary to heat coated glass sheets to temperatures at or near the softening point of glass (726 degrees C.), e.g., to temper the glass or enable it to be bent into desired shapes. Tempering is important for glass used in automobile windows, and particularly for glass used in automobile windshields, as well as in various architectural glazing applications. Upon breaking, tempered glass exhibits a break pattern in which the glass shatters into many small pieces, rather than into large dangerous shards. During tempering, coated glass is typically subjected to elevated temperatures on the order of about 700 degrees C. Moreover, the coated glass must be able to withstand such temperatures for substantial periods of time. Certain film stacks having silver as the infrared-reflective film are not able to withstand such high temperature processing without unacceptable deterioration of the silver film.
To avoid this problem, glass sheets can be heated (e.g., bent or tempered) before they are coated. The desired films can then be applied after heating. This procedure, however, tends to be complicated and costly and, more problematically, may produce non-uniform coatings.
In many cases, it is sufficient for temperable low-emissivity coatings to have only an upper blocker layer (i.e., without any lower blocker layer). In other cases, a reflective silver film is protected from deterioration at high temperatures by sandwiching the silver between two metallic blocker layers. In such cases, the two blocker layers are thick enough and reactive enough that when the coated glass is heated to high temperatures, these films capture oxygen and/or nitrogen that would otherwise reach and react with the silver.
It would be desirable to provide a low-emissivity coating based on a blocker layer material that can provide exceptional mechanical durability. It would be particularly desirable to provide a low-emissivity coating based on a blocker layer material that also provides exceptional moisture resistance, enables good control over optical properties (e.g., visible transmission), or both.